While the invention is primarily concerned with the practical methods of insect control, nevertheless the concepts here involved are applicable to all uses of controlled vapor dispersion. However, since insect attractants are of major importance, the description of the invention will be directed to such a usage.
That insects can be attracted or repelled by certain substances has been known for a great many years. In recent years, efforts have been made to utilize and extend this primitive knowledge for the purpose of controlling insect pests and mitigating the harmful effects they can inflict on man, animals, agricultural crops, clothing, etc. This has led to the recognition that olfaction plays a key role in communication among insects and that the media of communication are chemical substances produced and emitted by insects for communicative purposes. These chemical messengers have come to be called pheromones and are known to be highly specific as to insect species and elicited response. Pheromones may serve as alarm signals, food finding aids, mating signals, trail markers, or defensive agents for warding off predators.
A great many insect sex attractant pheromones have been isolated and identified as to chemical composition and structure. They typically are straight chain or cyclic organic compounds of carbon, hydrogen, and oxygen, falling in the molecular weight range of 150 to 300. The structure and biological activity of most of the known insect sex pheromones are extensively reviewed in the book, INSECT SEX PHEROMONES, by Martin Jacobson, Academic Press, New York, 1972. Of the identified insect sex pheromones, many have been synthesized and the synthetic materials used in a variety of ways to facilitate insect pest control.
Two general methods of employing insect sex pheromones have beemn applied. One method involves using the pheromone to attract a target insect to a trap or to a point where it can be destroyed by an insecticide. Trapping also serves as a survey means of timing the application of chemical insecticides. A second method involves broadcasting small point sources of pheromone over an infested area to disorient the insects and make it difficult or impossible for the opposite sexes to find one another for mating. This latter method is referred to as the discruption technique, and is intended to suppress the pest insect population by subverting or interrupting the natural mating and reproduction process.
Effective and economical use of insect sex pheromones for accomplishing insect pest control, by whatever strategy, requires a suitable means of dissemination. A dissemination method or device must accomplish discharge of the pheromone to the atmosphere at a specific rate peculiar to the target insect, and for whatever period of time the particular adult insects are active in mating. Since synthetic insect sex pheromones are frequently rather expensive materials, the dissemination means or devices must be as efficient as possible in utilization of pheromone. Thus, a practical and economical sustained release system must be employed for disseminating pheromone which provides for a controlled discharge of the attractant at a specified rate and for a specified period of time.
Numerous examples of sustained or controlled release systems exist in the prior art of insect pest control. U.S. Pat. Nos. 2,956,073, 3,116,201, and 3,318,769 teach the manufacture and use of insecticides formulated into shaped plastic articles which serve to release the insecticide at a prescribed rate over an extended period of time. U.S. Pat. No. 3,539,465 teaches the microencapsulation of hydrophilic liquid-in-oil emulsions in which polymeric capsular walls serve to mediate the controlled release of encapsulants such as insecticides and other biocidal agents. U.S. Pat. No. 3,740,419 teaches the use of insecticide impregnated wood chips as a slow release pest control device. U.S. Pat. No. 3,577,515 teaches the manufacture of microencapsulated insecticidal compositions by using interfacial polymerization to form a porous capsule wall which serves to regulate the rate of delivery of insecticide. U.S. Pat. No. 3,590,118 describes a slow release insect repellant system in the form of a breathable acrylic film. U.S. Pat. No. 3,592,910 discloses use of terpenoid resin-insecticide formulations which are designed to extend the period of effectiveness for nonpersistent or moderately persistent insecticides.
Each of these methods has its own particular cluster of advantages and drawbcks, which need not be elaborated here. Rather, it would be more in order to point out that researchers and economic entamologists continue to seek a more satisfactory scheme for the uniform, quantitatively predictable, prolonged automatic dissemination of miniscule amounts of active volatiles at an extremely low controlled rate. Sometimes the dissemination is desired on a tree-by-tree basis as, for example, in the control of certan orchards pests. Sometimes the dissemination is to be uniform and cover large areas, as in the use of pheromones to disrupt mating signals of insect pests attacking field crops. One possible superior means of achieving these ends entails the use of fine capillary tubes, both as containers and discharge devices.
Several researchers have had more or less limited success in the use of micro-tubes or micro-capillaries in laboratory tests and under limited experimental field conditions. We have encountered, and indeed have other workers in this area, several significant obstacles to the widespread practical applicability of the conventional forms of these devices. We have also discovered, however, a novel and superior variation of the micro-tube dissemination method and device which overcomes these obstacles and provides an extremely useful manner of using fiber tubes for the intended purpose in practical applications.
Previous users of micro-tubes for pheromone delivery have regularly employed them as containers and dispensers with both ends of the tube open to the atmosphere. The rate of pheromone discharged from the ends of the micro-tubes or micro-capillaries when used this way has generally been excessively high for many practical field applications. Moreover, when open at both ends such devices are subject to high material losses due to mechanical shock, vibration, wind, and the like.
Our invention, which comprises the use of micro-tubes or capillary channels of microconduits sealed at one end, while exceedingly simple, eliminates several significant obstacles to the use of such devices and confers on them practicability to an extent not heretofore contemplated by those familiar with the art.
As an example of the prior art, reference may be drawn to the article, "NOVEL TRAPPING AND DELIVERY SYSTEMS FOR AIRBORNE INSECT PHEROMONES," by Lloyd E. Browne, et al, J. Insect Physiol., 1974, Vol. 20, pp. 183 to 193. On Pages 187-188, a laboratory testing scheme for assaying pheromone efficiency is described where the active liquid is filled into a 5.mu.1 glass capillary mounted vertically and open at both ends. The contained liquid is continuously evaporated from the liquid-air interface exposed at the bottom of the capillary to which it continuously feeds by the action of gravity. While the delivery rate may be kept quite constant, it is also quite high, being of the order of 1 .mu.1/minute. The dimension of the capillary tube is about 5 cm long by 0.4 mm diameter. A full charge of hexane will be discharged in about 5 minutes in the test described.
Another example of the prior art using microtubes is described by Shorey et al, in "SEX PHEROMONES OF LEPIDOPTERA. XXX. DISRUPTION OF SEX PHEROMONE COMMUNCIATIONS IN `TRICHOPLASIA NI` AS A POSSIBLE MEANS OF MATING CONTROL," Environmental Entomology, Vol. 1, No. 5, October 1972, pp. 641-645. Schemes for evaporation of pheromones at lower, intermediate, and higher rates are discussed. It should be noted that in this work the authors view the use of micro-tubes as part of the higher rate technique.
"The substrates for higher evaporation rates were based on the principles of a liquid film of the pure chemical being exposed to the air. The rate was varied by varying the area of exposed flm. These evaporators could be left in the field for several days without a decrease in their release rate. In practice, however, they were serviced and recharged daily. The 10 ng/min evaporator consisted of an 0.38 mm ID Teflon tube, 20 mm long, held vertically in a clip attached to the wooden stake. Looplure was held in the lower end of the tube by capillarity. An inverted aluminum weighing cup attached to the top of the stake was used to shield the tube from excessive wind, which at times forced the looplure out of unshielded tubes. The 30 ng/min evaporator consisted of three similar Teflon tubes held in a single clip. "
The dissemination principle of this scheme is the same as that from the previous citation: liquid continously evaporates at a liquid-air interface maintained by gravity at the bottom of a double open-end vertical capillary tube. The particular dissemination rates for those materials are at least an order of magnitude higher and up to three orders of magnitude higher than can be achieved by the use of our invention. It will also be noted that because of the double open-end configuration, the tube contents are unstably retained and subject to being blown out by wind. Note, also, the requirement for daily attention. Both of these objectionable situations are obviated by our device.
As a further example of the prior art, we may cite Pitman and Vite, "FIELD RESPONSE OF DENDROCTONUS PSEUDOSUGAE (COLEOPTERA: SCOLYTIDAE) TO SYNTHETIC FRONTALIN," Annals of The Entomogical Society of America, Vol. 63, No. 3, pp. 661- 664, May 1970, who used 0.4 mm ID glass capillary tubes for the dissemination of synthetic frontalin. The release rate that they experienced was 5 mg/hr. By the use of our invention, with the same pheromone and approximately equivalent size tubing, the rate was reduced by two to three orders of magnitude (i.e., 100 to 1,000 times). As a result, the practical feasibility of our scheme to long-term controlled dissemination of minute amounts of this and other pheromone materials is superior; so much so as to render our method a practical scheme for commercial pest management, in contrast to the limited experimental activities of the prior workers.